The short answer is no. M-Go was designed to be a single security appliance per network, and the subscriptions would apply to a single unit on that network. If you want a different way to look at your design then you could create your network A with all the VLANs as it is today, then create your VLANs for network be and block them from the VLANs from network A. Lastly, you will go to network B and create your VLANS and SSIDs on network B. This could also reduce the need for ISP B. But I am assuming a lot in this option.
Your other option is to purchase a second subscription, and prepare to migrate from your GX20 at some point as it is on the EOL/EOS list.